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Section (2) syslog

Linux manual pages Section 2  

Name

syslog, klogctl — read and/or clear kernel message ring buffer; set console_loglevel

Synopsis

int syslog( int type,
  char *bufp,
  int len);
  
                 /* No wrapper provided in glibc */

/* The glibc interface */
#include <sys/klog.h>
int klogctl( int type,
  char *bufp,
  int len);
 

DESCRIPTION

[Note] Note

Probably, you are looking for the C library function syslog(), which talks to syslogd(8); see syslog(3) for details.

This page describes the kernel syslog() system call, which is used to control the kernel printk() buffer; the glibc wrapper function for the system call is called klogctl().

The kernel log buffer

The kernel has a cyclic buffer of length LOG_BUF_LEN in which messages given as arguments to the kernel function printk() are stored (regardless of their log level). In early kernels, LOG_BUF_LEN had the value 4096; from kernel 1.3.54, it was 8192; from kernel 2.1.113, it was 16384; since kernel 2.4.23/2.6, the value is a kernel configuration option (CONFIG_LOG_BUF_SHIFT, default value dependent on the architecture). Since Linux 2.6.6, the size can be queried with command type 10 (see below).

Commands

The type argument determines the action taken by this function. The list below specifies the values for type. The symbolic names are defined in the kernel source, but are not exported to user space; you will either need to use the numbers, or define the names yourself.

SYSLOG_ACTION_CLOSE (0)

Close the log. Currently a NOP.

SYSLOG_ACTION_OPEN (1)

Open the log. Currently a NOP.

SYSLOG_ACTION_READ (2)

Read from the log. The call waits until the kernel log buffer is nonempty, and then reads at most len bytes into the buffer pointed to by bufp. The call returns the number of bytes read. Bytes read from the log disappear from the log buffer: the information can be read only once. This is the function executed by the kernel when a user program reads /proc/kmsg.

SYSLOG_ACTION_READ_ALL (3)

Read all messages remaining in the ring buffer, placing them in the buffer pointed to by bufp. The call reads the last len bytes from the log buffer (nondestructively), but will not read more than was written into the buffer since the last clear ring buffer command (see command 5 below)). The call returns the number of bytes read.

SYSLOG_ACTION_READ_CLEAR (4)

Read and clear all messages remaining in the ring buffer. The call does precisely the same as for a type of 3, but also executes the clear ring buffer command.

SYSLOG_ACTION_CLEAR (5)

The call executes just the clear ring buffer command. The bufp and len arguments are ignored.

This command does not really clear the ring buffer. Rather, it sets a kernel bookkeeping variable that determines the results returned by commands 3 (SYSLOG_ACTION_READ_ALL) and 4 (SYSLOG_ACTION_READ_CLEAR). This command has no effect on commands 2 (SYSLOG_ACTION_READ) and 9 (SYSLOG_ACTION_SIZE_UNREAD).

SYSLOG_ACTION_CONSOLE_OFF (6)

The command saves the current value of console_loglevel and then sets console_loglevel to minimum_console_loglevel, so that no messages are printed to the console. Before Linux 2.6.32, the command simply sets console_loglevel to minimum_console_loglevel. See the discussion of /proc/sys/kernel/printk, below.

The bufp and len arguments are ignored.

SYSLOG_ACTION_CONSOLE_ON (7)

If a previous SYSLOG_ACTION_CONSOLE_OFF command has been performed, this command restores console_loglevel to the value that was saved by that command. Before Linux 2.6.32, this command simply sets console_loglevel to default_console_loglevel. See the discussion of /proc/sys/kernel/printk, below.

The bufp and len arguments are ignored.

SYSLOG_ACTION_CONSOLE_LEVEL (8)

The call sets console_loglevel to the value given in len, which must be an integer between 1 and 8 (inclusive). The kernel silently enforces a minimum value of minimum_console_loglevel for len. See the log level section for details. The bufp argument is ignored.

SYSLOG_ACTION_SIZE_UNREAD (9) (since Linux 2.4.10)

The call returns the number of bytes currently available to be read from the kernel log buffer via command 2 (SYSLOG_ACTION_READ). The bufp and len arguments are ignored.

SYSLOG_ACTION_SIZE_BUFFER (10) (since Linux 2.6.6)

This command returns the total size of the kernel log buffer. The bufp and len arguments are ignored.

All commands except 3 and 10 require privilege. In Linux kernels before 2.6.37, command types 3 and 10 are allowed to unprivileged processes; since Linux 2.6.37, these commands are allowed to unprivileged processes only if /proc/sys/kernel/dmesg_restrict has the value 0. Before Linux 2.6.37, privileged means that the caller has the CAP_SYS_ADMIN capability. Since Linux 2.6.37, privileged means that the caller has either the CAP_SYS_ADMIN capability (now deprecated for this purpose) or the (new) CAP_SYSLOG capability.

/proc/sys/kernel/printk

/proc/sys/kernel/printk is a writable file containing four integer values that influence kernel printk() behavior when printing or logging error messages. The four values are:

console_loglevel

Only messages with a log level lower than this value will be printed to the console. The default value for this field is DEFAULT_CONSOLE_LOGLEVEL (7), but it is set to 4 if the kernel command line contains the word quiet, 10 if the kernel command line contains the word debug, and to 15 in case of a kernel fault (the 10 and 15 are just silly, and equivalent to 8). The value of console_loglevel can be set (to a value in the range 1–8) by a syslog() call with a type of 8.

default_message_loglevel

This value will be used as the log level for printk() messages that do not have an explicit level. Up to and including Linux 2.6.38, the hard-coded default value for this field was 4 (KERN_WARNING); since Linux 2.6.39, the default value is a defined by the kernel configuration option CONFIG_DEFAULT_MESSAGE_LOGLEVEL, which defaults to 4.

minimum_console_loglevel

The value in this field is the minimum value to which console_loglevel can be set.

default_console_loglevel

This is the default value for console_loglevel.

The log level

Every printk() message has its own log level. If the log level is not explicitly specified as part of the message, it defaults to default_message_loglevel. The conventional meaning of the log level is as follows:

Kernel constant Level value Meaning
KERN_EMERG 0 System is unusable
KERN_ALERT 1 Action must be taken immediately
KERN_CRIT 2 Critical conditions
KERN_ERR 3 Error conditions
KERN_WARNING 4 Warning conditions
KERN_NOTICE 5 Normal but significant condition
KERN_INFO 6 Informational
KERN_DEBUG 7 Debug-level messages

The kernel printk() routine will print a message on the console only if it has a log level less than the value of console_loglevel.

RETURN VALUE

For type equal to 2, 3, or 4, a successful call to syslog() returns the number of bytes read. For type 9, syslog() returns the number of bytes currently available to be read on the kernel log buffer. For type 10, syslog() returns the total size of the kernel log buffer. For other values of type, 0 is returned on success.

In case of error, −1 is returned, and errno is set to indicate the error.

ERRORS

EINVAL

Bad arguments (e.g., bad type; or for type 2, 3, or 4, buf is NULL, or len is less than zero; or for type 8, the level is outside the range 1 to 8).

ENOSYS

This syslog() system call is not available, because the kernel was compiled with the CONFIG_PRINTK kernel-configuration option disabled.

EPERM

An attempt was made to change console_loglevel or clear the kernel message ring buffer by a process without sufficient privilege (more precisely: without the CAP_SYS_ADMIN or CAP_SYSLOG capability).

ERESTARTSYS

System call was interrupted by a signal; nothing was read. (This can be seen only during a trace.)

CONFORMING TO

This system call is Linux-specific and should not be used in programs intended to be portable.

NOTES

From the very start, people noted that it is unfortunate that a system call and a library routine of the same name are entirely different animals.

SEE ALSO

dmesg(1), syslog(3), capabilities(7)

COLOPHON

This page is part of release 4.16 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man−pages/.


  t
Copyright (C) 1995 Andries Brouwer (aebcwi.nl)
and Copyright (C) 2012, 2014 Michael Kerrisk <mtk.manpagesgmail.com>

%%%LICENSE_START(VERBATIM)
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working
professionally.

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.
%%%LICENSE_END

Written 11 June 1995 by Andries Brouwer <aebcwi.nl>
2008-02-15, Jeremy Kerr <jkozlabs.org>
    Add info on command type 10; add details on types 6, 7, 8, & 9.
2008-02-15, Michael Kerrisk <mtk.manpagesgmail.com>
    Update LOG_BUF_LEN details; update RETURN VALUE section.

Section (3) syslog

Linux manual pages Section 3  

Name

closelog, openlog, syslog, vsyslog — send messages to the system logger

Synopsis

#include <syslog.h>
void openlog( const char *ident,
  int option,
  int facility);
 
void syslog( int priority,
  const char *format,
  ...);
 
void closelog( void);  
 
void vsyslog( int priority,
  const char *format,
  va_list ap);
 
[Note] Note
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
vsyslog():
Since glibc 2.19:
_DEFAULT_SOURCE Glibc 2.19 and earlier:
_BSD_SOURCE

DESCRIPTION

openlog()

openlog() opens a connection to the system logger for a program.

The string pointed to by ident is prepended to every message, and is typically set to the program name. If ident is NULL, the program name is used. (POSIX.1-2008 does not specify the behavior when ident is NULL.)

The option argument specifies flags which control the operation of openlog() and subsequent calls to syslog(). The facility argument establishes a default to be used if none is specified in subsequent calls to syslog(). The values that may be specified for option and facility are described below.

The use of openlog() is optional; it will automatically be called by syslog() if necessary, in which case ident will default to NULL.

syslog() and vsyslog()

syslog() generates a log message, which will be distributed by syslogd(8).

The priority argument is formed by ORing together a facility value and a level value (described below). If no facility value is ORed into priority, then the default value set by openlog() is used, or, if there was no preceding openlog() call, a default of LOG_USER is employed.

The remaining arguments are a format, as in printf(3), and any arguments required by the format, except that the two-character sequence %m will be replaced by the error message string strerror(errno). The format string need not include a terminating newline character.

The function vsyslog() performs the same task as syslog() with the difference that it takes a set of arguments which have been obtained using the stdarg(3) variable argument list macros.

closelog()

closelog() closes the file descriptor being used to write to the system logger. The use of closelog() is optional.

Values for option

The option argument to openlog() is a bit mask constructed by ORing together any of the following values:

LOG_CONS

Write directly to the system console if there is an error while sending to the system logger.

LOG_NDELAY

Open the connection immediately (normally, the connection is opened when the first message is logged). This may be useful, for example, if a subsequent chroot(2) would make the pathname used internally by the logging facility unreachable.

LOG_NOWAIT

Don_zsingle_quotesz_t wait for child processes that may have been created while logging the message. (The GNU C library does not create a child process, so this option has no effect on Linux.)

LOG_ODELAY

The converse of LOG_NDELAY; opening of the connection is delayed until syslog() is called. (This is the default, and need not be specified.)

LOG_PERROR

(Not in POSIX.1-2001 or POSIX.1-2008.) Also log the message to stderr.

LOG_PID

Include the caller_zsingle_quotesz_s PID with each message.

Values for facility

The facility argument is used to specify what type of program is logging the message. This lets the configuration file specify that messages from different facilities will be handled differently.

LOG_AUTH

security/authorization messages

LOG_AUTHPRIV

security/authorization messages (private)

LOG_CRON

clock daemon (cron and at)

LOG_DAEMON

system daemons without separate facility value

LOG_FTP

ftp daemon

LOG_KERN

kernel messages (these can_zsingle_quotesz_t be generated from user processes)

LOG_LOCAL0 through LOG_LOCAL7

reserved for local use

LOG_LPR

line printer subsystem

LOG_MAIL

mail subsystem

LOG_NEWS

USENET news subsystem

LOG_SYSLOG

messages generated internally by syslogd(8)

LOG_USER (default)

generic user-level messages

LOG_UUCP

UUCP subsystem

Values for level

This determines the importance of the message. The levels are, in order of decreasing importance:

LOG_EMERG

system is unusable

LOG_ALERT

action must be taken immediately

LOG_CRIT

critical conditions

LOG_ERR

error conditions

LOG_WARNING

warning conditions

LOG_NOTICE

normal, but significant, condition

LOG_INFO

informational message

LOG_DEBUG

debug-level message

The function setlogmask(3) can be used to restrict logging to specified levels only.

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
openlog(), closelog() Thread safety MT-Safe
syslog(), vsyslog() Thread safety MT-Safe env locale

CONFORMING TO

The functions openlog(), closelog(), and syslog() (but not vsyslog()) are specified in SUSv2, POSIX.1-2001, and POSIX.1-2008.

POSIX.1-2001 specifies only the LOG_USER and LOG_LOCAL* values for facility. However, with the exception of LOG_AUTHPRIV and LOG_FTP, the other facility values appear on most UNIX systems.

The LOG_PERROR value for option is not specified by POSIX.1-2001 or POSIX.1-2008, but is available in most versions of UNIX.

NOTES

The argument ident in the call of openlog() is probably stored as-is. Thus, if the string it points to is changed, syslog() may start prepending the changed string, and if the string it points to ceases to exist, the results are undefined. Most portable is to use a string constant.

Never pass a string with user-supplied data as a format, use the following instead:

syslog(priority, %s, string);

SEE ALSO

journalctl(1), logger(1), setlogmask(3), syslog.conf(5), syslogd(8)

COLOPHON

This page is part of release 4.16 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man−pages/.


  Written  Feb 1994 by Steve Greenland (stevegrneosoft.com)
and Copyright 2001, 2017 Michael Kerrisk <mtk.manpagesgmail.com>

%%%LICENSE_START(VERBATIM)
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working
professionally.

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.
%%%LICENSE_END

Updated 1999.12.19 by Karl M. Hegbloom <karlhegdebian.org>

Updated 13 Oct 2001, Michael Kerrisk <mtk.manpagesgmail.com>
Added description of vsyslog
Added descriptions of LOG_ODELAY and LOG_NOWAIT
Added brief description of facility and option arguments
Added CONFORMING TO section
2001-10-13, aeb, minor changes
Modified 13 Dec 2001, Martin Schulze <joeyinfodrom.org>
Modified 3 Jan 2002, Michael Kerrisk <mtk.manpagesgmail.com>